Baror* Humboldt on Rock Formations, 
of colour. At Mexico, I have seen dark greenstones, al- 
ternate thousands of times with reddish-white syenites, abound- 
ing more in quartz than in felspar. In this greenstone there 
were veins of syenite, and in the syenite veins of' greenstone ; 
but none of the two rocks passed into the other. (Essaipolu 
tique sur la Nouvelle Espagne^ v. ii. p. 523. J They present at 
the limit of their mutual contact, differences as marked as the 
porphyries which alternate with the greywackes or with the sye- 
nites, as the black limestones which a»lternate with the transition 
clay-slates, and so many other rocks of entirely heterogeneous 
composition and aspect. Further, when, in primitive deposites, 
rocks more related by the nature of their composition than by 
their structure or mode of aggregation, for example, the granites 
and gneisses, or the gneisses and mica-schists, alternate ; these 
rocks do not by any means show the same tendency to pass in- 
to each other, as they present, when isolated in formations which 
are not of a complex character. We have already observed, that 
often a bed /S, becoming more frequent in the rock «, announces 
to the traveller that the simple formation ««, is to be succeeded 
by a compound formation, in which « and /3 alternate. Farther 
on, it happens, that assumes a greater development ; that «. is 
no longer an alternating rock, but a simple bed subordinate to /8, 
and that this rock /S shows itself alone, until, by the frequent 
repetition of beds y, it becomes the precursor of a compound for- 
mation of /3, alternating with y. We might substitute for these 
signs the words granite, gneiss, and mica-slate ; those of por- 
phyry, greywacke and syenite ; of gypsum, marl and fetid lime- 
stone (stinkstein). Pasigraphic language has the advantage of 
generalizing the problems ; it is more conformable to the wants 
of geognostical philosophy of which I attempt to present here 
the first elements, in so far as they have relation to the study of 
the superposition of rocks. Now, if often between formations 
which are simple and very closely allied, in the order of their 
relative antiquity, between the formations y, there occur 
, compound formations interposed, <«/3 and /3y, (that is to say a. al- 
ternating with /3, and /3 alternating with y) ; we observe, also, 
although less frequently, that a formation (for example «,) as- 
sumes so extraordinary an increase, that it envelopes the forma- 
VOL, X. NO. 20 . APRIL 1824 . R 
