HISTORY OF CRYSTALLINE ROCKS. 29 
taught the igneous origin of certain limestones—an opinion afterwards adopted by J. D. 
Dana, who supposed that some of the so-called primary limestones “were of igneous 
origin, like granite.” The aqueous origin of similar calcareous masses in Scandinavia had, 
however, been recognized by Scheerer and by Daubrée, and in Germany by Bischof ; 
while the vein-like character of certain aggregates of this kind, in which various silicates 
and other mineral species are associated with carbonate of lime in the ancient gneisses of 
North America, had been noticed by C. U. Shepard, H. D. Rogers, and W. P. Blake, among 
others, as was shown by the writer in some detail, in 1866. In a paper then read before 
the American Association for the Advancement of Science, it was said that deposits of 
carbonate of lime, sometimes of great dimensions, and holding the characteristic minerals 
of the crystalline limestones, are found filling fissures and veins in the Laurentian gneisses. 
These were then designated endogenous rocks, regarded as of aqueous origin, and to be 
carefully distinguished from intrusive or exotic rocks." 
The subject was discussed in the same year in an account of the mineralogy of the 
Laurentian rocks, when it was said, in commenting upon the view of Emmons that such 
masses, and in fact all of the crystalline limestones of the series, are eruptive : — “The 
greater part of the calcareous rocks in the Laurentian system in North America are 
stratified, and the so-called eruptive limestones are really calcareous veinstones or endogen- 
ous rocks, generally including foreign minerals, such as pyroxene, scapolite, orthoclase, 
quartz, ete.”* I had not at that time as yet discovered that these same endogenous 
masses may include, besides calcareous bands, others essentially quartzose, pyroxenic, 
and feldspathic, resembling more or less the strata of the enclosing gneissic series, nor con- 
sidered that these same calcareous bands might sometimes be found in fissures coincident 
with the bedding of the latter. 
§ 52. In 1869 I visited with the late L. 8. Burbank a locality at Chelmsford, Massa- 
chusetts, where limestone has been quarried from interrupted masses, sometimes two 
hundred feet in length, enclosed in gneisses of the ordinary Laurentian type, to which I 
then referred them.” I failed to recognize in the quarries then examined the endogenous 
character which doubtless belongs to some of the limestone-masses of this region, where 
they have been traced, at intervals, for twenty-five miles, through Chelmsford, Boxbor- 
ough, and Bolton; but at the same time I called the attention of Mr. Burbank to the 
question of these vein-like masses, placing in his hands my publications of 1866. He, as a 
result of farther observations, had, in 1871, persuaded himself that all of the limestones of 
the region were newer than the enclosing rocks, not eruptive, but “of a vein-like char- 
acter,” occupying fissures in the gneiss, of which character his descriptions, in certain 
cases, give evidence. * He noted the banded structure visible in the arrangement of the 
various enclosed minerals, as I had described them in 1866 in the similar limestone- 
masses in Canada, and enumerated in the region under consideration : amphibole, pyr- 
oxene, chrysolite (forsterite or so-called boltonite), phlogopite, scapolite, and garnet, 
besides serpentine, in grains or in irregular bands or layers, sometimes traversed by veins 
of SOU To this list may be added spinel: chondrodite, perlite, and titanite. 
' Proc. Amer. Assoc. Adv. Science, 1866, p. we also Can. Natural (IL.) iii. 123, 
* Report Geol. Survey of Canada, 1863-66, p. 194. See also the facts resumed in Chem. and Geol. Essays, p. 218. 
* Amer. Jour. Science, 1870, xlix. 75. 
* Burbank. Proc. Amer. Assoc. Ady. Science, 1871, pp. 263-266. 
