ERUPTIVE EOCKS. 297 



The Archean exposures studied in this region were once covered by 

 portions of the same sedimentary series in which these intrusive sheets 

 are now found. It may, therefore, be assumed that the form of the chan- 

 nels through which the fused masses were forced up into the overlying 

 beds is fairly represented by the average outline of these dikes. According 

 to this reasoning it is evident that the channels in the Archean were ex- 

 tremely small as compared with the extent of the sheets themselves, and were 

 rather in the form of the narrow fissure, which has been supposed to be the 

 source of so-called massive eruptions, than of the rounded "necks," which 

 have sometimes been observed as the actual vents of volcanic eruptions. 



Relation of form to composition. In comparing the relative form of the in- 

 trusive bodies with the composition and structure of the rocks which com- 

 pose them, it is found that the more basic the rock the thinner is the sheet 

 and the greater its relative extent in a horizontal direction. It is true 

 the range of relative acidity in this region is not the very widest, the White 

 Porphyry, whose beds are relatively the thickest, having about 70 per cent, 

 of silica, while the hornblende-porphyrite, which occurs in the thinnest sheets 

 and at the same time with relatively great horizontal extension, has a little 

 over 56 per cent, of silica. Basalts range from 45 to 50 per cent, of silica. 

 . The great sheet of White Porphyry has an estimated thickness of 1,500 feet at 

 the point of its supposed intrusion from below, and the least thickness ob- 

 served is 20 feet. The Sacramento Porphyry, which is slightly less acid 

 (having 65 per cent, of silica) and whose greatest thickness is found im- 

 mediately adjoining the White Porphyry laccolite, is evidently somewhat 

 thinner, being probably less than 1,000 feet at its maximum, while the horn- 

 blende-porphyrite sheet of Mosquito and Buckskin gulches is found in a 

 maximum thickness of 25 feet, and frequently thins to 6 feet, or even less; 

 yet its practical continuity over considerable areas is even more readily 

 evident than in the case of the larger masses of quartz-porphyry. A more 

 remarkable instance of the great relative area of a basic intrusive sheet is 

 that of the Whin Sill, in Northumberland, England, which, according to 

 Messrs. Topley and Lebour, 1 is a basaltic intrusive sheet, that has been traced 

 with unimportant breaks for a distance of 75 to 80 miles in a thickness of 



'Quarterly Journal of tho Geological Society, XXXIII, pp. 406-421, 1877. 



