MUSEUM OF COMPARATIVE ZOOLOCY. 233 



mass of the rock, and eruptive through the conglomerate. In the 

 second place, a close study of the great area of this rock shows that 

 it is cut by a large number of narrow diabase dikes, generally but a 

 few feet in width (e. g. three feet), which do not have a marked amyg- 

 daloidal structure. In many cases they run almost parallel with the 

 trend of the amygdaloid ; in other cases they run obliquelj', while 

 others again may cut almost transversely across it, in parallelism witli 

 a third dike to be mentioned. These dikes show generally well-marked 

 contacts with the amygdaloid : they are fine-grained at the junction, 

 but coarse-grained in the centre ; in some cases they have melted 

 the amygdaloid at the contact, so that it is difficult to find the actual 

 line. Some of them are easily distinguished from the amygdaloid, un- 

 der the microscope, by the large amount of augite which they contain, 

 but in others this mineral cannot be found as such, for all trace of it 

 (if originally present) has been lost in the alteration products. Lastly, 

 about the middle of the large western exposure of the amygdaloid there 

 is found a large dike (at least seventy feet "wide) running transversely 

 across it, in a direction N. 5°-10° W. On the south side it is seen 

 breaking through the conglomerate and sandstone, and can be traced 

 from that locality across the amj-gdaloid. No exposure was found giving 

 the actual contact of this rock with any of the others, although it can 

 be seen at a distance of a foot or two from them. Judging from its 

 direction, which cuts directly across the trend of a great number of the 

 small dikes, it would seem to be the latest rock of all. Some of the 

 small dikes to the east of it are, however, nearly parallel with it. While, 

 therefore, there seem to be at least two periods of eruption subsequent 

 to that of the amygdaloid, yet some of these small dikes may cut the 

 others, thus complicating the phenomena still furthei*. T have not been 

 able to find evidence of this bej^ond the difference of direction; and to 

 settle the question by the discovery of the actual contacts will be diffi- 

 cult, on account of the lithological similarity of all the eruptive rocks, 

 and the thick covering of lichens, which conceals everything, and makes 

 any work there laborious. 



The amygdaloid sometimes loses its amygdaloidal character, so as 

 to resemble greatly the later dikes ; but in such cases the passage is 

 gradual. 



The microscopic descriptions which follow show that all these rocks 

 are altered basalts, and, together with the field relations, prove thai 

 they are all truly eruptive rocks, breaking through the conglomerate, 

 while the later eruptive rocks cut the earlier ones. 



