540 AUSTIN F. ROGERS 



fact, fossil resins and fossil fuels. They are collected and studied 

 by geologists. 



The hydrocarbons are by some mineralogists given full rank as 

 minerals, by others omitted entirely, by still others treated in an 

 appendix to minerals. The latter procedure seems to be the safest 

 plan, for they are organic substances, and the typical minerals are 

 certainly inorganic, yet on the other hand they deserve some recog- 

 nition by the mineralogist. The mineralogist is often called upon 

 to identify them, and they should be described from a mineralogical 

 standpoint. 



For these reasons I think the hydrocarbons may be included 

 under Niedzwiedzki's term miner aloid. 1 As Niedzwiedzki used 

 this term for all naturally occurring amorphous substances, this 

 changes somewhat the original definition of mineraloid. Such 

 substances as opal, cliachite, limonite, collophane, halloysite, etc., 

 are definite enough to be called minerals even though they are 

 amorphous. The term "mineraloid" seems appropriate for the 

 less definite mineral-like substances. 



And in the same category I would also place the natural glasses. 

 As far as I know, glass is not given a place in any modern mineral- 

 ogical treatise, although the older mineralogists described tachylyte 

 and hyalomelane as mineral species. It is, however, included in 

 some of the determinative tables of the rock-forming minerals 

 found in textbooks on petrography, and for the same reason that it 

 is included in these tables it may be treated as a mineraloid. 

 Natural glass is a homogeneous substance to be identified the same 

 as minerals in general. 



Natural glass is, of course, varied in composition in comparison 

 with the various types of igneous rocks, yet the average obsidian is 

 probably not much more varied than some of the amorphous miner- 

 als. While glass is scarcely entitled to recognition as a mineral, 

 there are arguments in favor of classifying it as a mineraloid. It 

 may well receive a place in the appendixes of our books on miner- 

 alogy. This will call attention to its properties and will aid in its 

 identification, which otherwise might be difficult for a beginner who 

 had studied mineralogy but not petrography. 



1 Centralblatt f. Min., Geol., u. Pal., 1909, pp. 661-63. 



