122 GEOLOGY OF THE HIGH PLATEAUS. 



the lavas have emanated from primordial rocks, what are they % There is 

 one great, group of lavas which quickly furnish ground for suspicion. 



Recurring here to the generalization that the materials composing the 

 stratified rocks have been ultimately derived from primordial matter, it is 

 but an identical proposition to say that the chemical constitution of that 

 primordial matter ought inferentially to be such as would yield the mate- 

 rials of the sedimentary rocks. It ought to possess the same constituents, 

 and ought also to contain them in substantially the same proportions as the 

 average constitution of the stratified rocks taken as a whole category. In 

 a word, it should be what some biologists might call a synthetic or compre- 

 hensive type of rock, from which the stratified materials might be differ- 

 entiated by the known processes of sub-aerial decomposition and selection. 

 Secondly, it ought not to conform in composition to any one variety of 

 stratified rock, unless, perchance, in some rare exceptional cases. Thirdly, 

 it ought to be a very abundant and voluminous rock, erupted at almost any 

 geological age or period, from the present as far back into the past as we 

 are able to discriminate the age of an eruption. Among the several groups 

 or sub-groups of volcanic rocks do we find any one of them answering to 

 this ideal type ? This question does not admit of a very brief and decisive 

 answer. We have no very accurate knowledge of the mean constitution 

 of the stratified rocks. There is a statement, handed down, I believe, from 

 Bischof, and passing current in the text-books, that silica constitutes very 

 nearly 50 per cent, of the mass of all known rocks, and the estimate seems 

 to be a very fair one. Its probable error is certainly small if the impres- 

 sions of the geologists who have given much attention to lithology are to 

 be trusted. This percentage of silica is substantially the same as that 

 found in the basalts, and if there be a synthetic type of eruptive rocks 

 this fact fastens suspicion at once upon the basaltic group. Probably no 

 lilhologist will hesitate to say that next to silica the most abundant con- 

 stituent of the stratified rocks is alumina ; but the exact proportions we do 

 not know. Alumina is, however, known to be the second in quantity in 

 the constitution of average basalt. But the third constituent of basalt in 

 respect to quantity is iron oxide ; in the foliated rocks it is unquestionably 

 lime. Here is a discrepancy, and a well-marked one, which we cannot 



