Ragnarok : the age of fire and gravel

——EE EEE

WHAT IS A COMET? 75

“The aluminous minerals contained in granite rocks are feldspar, mica, and hornblende. . . . Mica and hornblende generally contain considerable oxide of iron, while feldspar usually yields only a trace or none. Therefore clays which are derived from feldspar are light-colored or white, while those partially made up of decomposed mica or hornblende are dark, either bluish or red.” *

The tail of the comet seems to be perpetually in motion. It is, says one writer, “continually changing and Jjluctuating as vaporous masses of cloud-like structure might be conceived to do, and in some instances there has been a strong appearance even of an undulating movement.” ¢

The great comet of 1858, Donati’s comet, which many now living will well remember, and which was of such size that when its head was near our horizon the extremity of the tail reached nearly to the zenith, illustrated this continual movement of the material of the tail ; that appendage shrank and enlarged millions of miles in length.

Mr. Lockyer believed that he saw in Coggia’s comet the evidences of a whirling motion—

“Tn which the regions of greatest brightness were caused by the different coils cutting, or appearing to cut, each other, and so in these parts leading to compression

or condensation, and frequent collision of the luminous particles.”

Olbers saw in a comet’s tail—

“A sudden flash and pulsation of light which vibrated for several seconds through it, and the tail appeared during the continuance of the pulsations of light to be lenethened by several degrees and then again contracted,” *

* “ American Cyclopedia,” article “ Clay.” + “Edinburgh Review,” October, 1874, p. 208. ¢ “ Cosmos,” vol. i, p. 143,