108 REPORT—1858. 
‘which crystals of a far less fusible mineral have been formed. Thus, for instance, in 
the lava of Vesuvius crystals of Jeucite have very often been deposited on nuclei of 
augite. If it were supposed that the temperature at which the minerals crystallized 
was the same as that of their own fusing-point when heated alone, this would appear 
to be a very unintelligible circumstance, but can easily be explained by supposing 
that the fused rock is simply a liquid melting at a high temperature, which is capable 
of dissolving various minerals, in the same manner that the very fusible liquid water 
dissolves various salts. If a solution of bichromate of potash, nearly saturated at the 
freezing-point of water, be slowly frozen, small crystals of ice are formed, and on 
these are deposited small erystals of bichromate of potash, which can no longer be held 
in solution in the diminished quantity of liguad water. Here then, as in the above- 
named instance of the arrangement of the crystals in lava, crystals of a very fusible 
substance act as the nuclei on which far less fusible crystals are deposited; and if 
we suppose that there is a perfect analogy between the two cases,—that both were 
liquids holding various substances in solution,—the arrangement of the minerals in the 
igneous rock no longer presents any special or exceptional peculiarity. 
On the Currents present during the Deposition of the Carboniferous and 
Permian Strata in South Yorkshire and North Derbyshire. By H.C. 
Sorsy, F.R.S. 
This was a continuation of a branch of geology on which the author has published 
several papers, pointing out how the direction and characters of the currents present 
during the deposition of stratified rocks can be determined (Report, 1855, p. 97). 
The neighbourhood of Sheftield is extremely well fitted for this inquiry ; and fromre- 
siding there the author has been able to examine minutely a large tract of country, 
and lay down the direction of the current in many hundred localities on large maps, 
which were exhibited. The chief conclusions derived from these are, that during the 
period of the millstone-grit there was a very uniform general current from the north- 
east, slightly interfered with by a tide setting from the north-west, and by the action 
of surface-waves and wind-drift currents produced by the powerful westerly gales, 
This general north-east current was also present during the deposition of the gritstone 
beds in the lower part of the coal strata, in the shales associated with which genuine 
marine shells are found, but ceased towards the central portion, more productive in 
coal, where such marine shells do not occur, In that part of the series, in different 
localities and beds, the currents were from all parts of the compass ; but on the whole 
are chiefly from the west, as if in some way or other connected with the prevailing 
westerly winds and wind-drift currents, which here prevailed independent of the tide 
or general north-east current, on account of the connexion with the main sea having 
been cut off. During the deposition of the magnesian limestone the sea appears to 
have been subject to a very decided tide, rising and falling with great uniformity from 
W.S.W. to E.N.E., amongst a number of shoals on which surface-waves stranded, 
chiefly produced by easterly winds. The millstone-grit and lower coal-measures 
therefore present us with an admirable example of the action of a simple current, 
flowing only in one direction; whereas the magnesian limestone is a very excellent 
illustration of the effect of oscillating currents moving backward and forward in a 
particular line, like the currents produced by the rise and fall of the tide. 
On the Geology of the Scilly Isles. 
By the Rev. Francis F. Stratuam, B.A., F.G.S. 
The author referred to the erroneous impressions which generally prevail with re- 
gard to the size, number and character of the Scilly group, which he described as 
presenting many features of interest independently of their curious geologic pheno- 
mena, It consists of a cluster of small islands or rocks 145 in number, varying in 
size from the mere solitary crag jutting out at low water from the surface of the ocean, 
to the Isle of St. Mary, which is the largest, the most populous, and the most fertile 
of the whole, measuring about 3 miles by 23, and containing an estimated area of 
about 1640 acres. They lie in latitude 49° 57’ N., and in longitude 6° 43! W., bear- 
ing W. by S, from Land’s End, and due W. from the Lizard, from the former of 
